A computer system may include one or more optical systems that provide an image as output or receive an image as input. Example optical systems include displays, cameras, scanners, and certain kinds of touch-sensitive input systems. Some optical systems may include a light guide that transmits an image to a touch-sensitive display surface, focuses an image on a detector, or does both. The light guide may be wedge-shaped, transparent in one or more visible and/or infrared wavelength ranges, and comprise at least one pair of opposing faces. Through the light guide, light of a certain wavelength range may propagate laterally, via internal reflection from the opposing faces. In many cases, the material properties and overall configuration of the light guide may affect the intensity and fidelity of the images provided by the optical system.